My eye is turning black when adding the transmission on the BSDF node
He's in Eevee, maybe that setup works fine in Eevee, but in Cycles I think you need at least to mix your Principled BSDF (with 1.00 Transmission) with a Transparent BSDF node. There is a lot of solutions (like use Fresnel in the Mix factor, use Glossy, etc) but it will always be a mix with Transparent BSDF in my opinion.